Use of glass ceramic plates as cooktops in electric cooking apparatus is becoming increasingly common. Among the advantages of this smooth cooking surface is its pleasing appearance and easy cleanability. However, due to the high thermal impedance of the glass ceramic plate, such cooktops are less efficient thermally than conventional cooking surfaces using sheathed heating elements.
Due to unique electrical and thermal characteristics possessed by materials such as molybdenum disilicide (MoSi.sub.2) and tungsten, resistive heating elements made from these materials are attractive for use with glass ceramic cooktops. The high positive temperature coefficient of resistivity, low thermal mass, and low specific heat characteristic of MoSi.sub.2 and tungsten and the high operating temperature achievable using heating elements made from these materials provide the potential for improved thermal efficiency for cooking apparatus which incorporate a glass ceramic cooktop. However, these same dynamic electrical and thermal characteristics create power control problems which have thus far rendered the use of heating elements made from these materials impractical in electric cooking apparatus.
Conventionally, power control in electric cooking apparatus is achieved using temperature sensitive switches, such as bimetalic infinite heat switches. In operation, the operator adjusts the switch to provide the desired cooking temperature. The switch remains closed until the heating element reaches a predetermined temperature. The switch then opens and remains open until the element temperature drops to a predetermined temperature. The switch continues to cycle ON and OFF in this manner indefinitely. Since conventional sheathed heating elements heat up and cool down relatively slowly, these switching cycles are relatively long, ranging from a few seconds to thirty seconds. In addition, the resistance of a conventional sheathed heating element changes only slightly in going from room temperature to operating temperature. Since the resistance of conventional heating elements is relatively independent of temperature in the temperature range of interest, transient current surges when the switches close are minimal. Thus, conventional power control techniques work satisfactorily.
However, the dynamic characteristics of resistive heating elements made from MoSi.sub.2 or tungsten prevent these heating elements from being controlled, using conventional control techniques. Firstly, a MoSi.sub.2 heating element, as described generally in U.S. Pat. No. 3,912,905, designed for use in a cooking appliance, typically varies in resistance from 2-3 ohms at room temperature to 25 ohms at an operating temperature of approximately 1000.degree. C. Thus, assuming energization from a standard 240 volt AC household supply, as the temperature of the heating element changes from room temperature to operating temperature, the load current changes from an initial peak of roughly 110 amps to a steady state current on the order of 8.5 amps RMS. This initial current of 110 amps is obviously greater than can be tolerated in a household appliance except for extremely brief periods. Secondly, the heating element cools extremely rapidly; the first time constant for thermal response of this heating element being in the 600-1000 millisecond range. Since the element cools rapidly with a concurrent drop in resistance, excessive current surges may occur even during steady state operation because the resistance of the element may drop between applications of power to a level which draws excessive current during each subsequent application of power. Therefore, a very rapid switching capability which enables the use of brief ON times to limit the duration of excessive current during the heat-up of the element and brief OFF times to prevent unacceptable drops in resistance during steady state operation by limiting cooling of the element between ON times is required to avoid frequent excessive current surges.
Clearly, the relatively slow mechanical switching of the conventionally employing infinite heat switches cannot provide the rapid switching required to prevent excessive current flow during each application of power. Similarly, conventional electronic controls for use with conventional heating elements in cooking appliances have been designed to employ relatively long ON and OFF periods.
This problem was addressed in concurrently-filed, commonly-assigned application Ser. No. 008,356, filed in the name of Thomas R. Payne, entitled "Power Control For Appliances Using High Inrush Current Element," which disclosure is hereby incorporated by reference. This application discloses a control system for use with a MoSi.sub.2 type heating element which varies the duty cycle of the heating element in response to operator selected power setting inputs. The duty cycle control system disclosed therein provides a solution to the steady state current surge problem by limiting the duration of the maximum idle time between power ON times to substantially less than one heating element thermal time constant. This duty cycle control approach works satisfactorily provided the control period is short enough to prevent substantial cooling of the heating element betweem ON periods. However, the control period of the control system disclosed in the above-noted application allows only a limited range of cooking temperatures. In order to expand the range of cooking temperatures to include an acceptable selection of lower temperatures, a longer control period than that employed in the system disclosed in the above-noted application is required. This requirement is aggravated in the case of MoSi.sub.2 type heating elements because of the non-linear relationship between ON time and heating element output power which, particularly at the lower power settings, results in output powers which are substantially greater relative to ON time than is typical of conventional resistance heating elements in which ON time is essentially linearly related to output power.
Although the control periods as mentioned above can be extended to one or two time constants without causing steady state current surges sufficiently in excess of household limits to trigger circuit breakers, the resulting recurring current surges may significantly reduce the reliability of the circuit components, particularly the solid state switching devices as the duration of the OFF time approaches several time constants. For example, for a control period extended to include 64 half-cycles (approximately 1/2 second), even at the relatively high end of the output power duty cycle range such as 50% duty cycle, i.e. 32 successive power On half-cycles followed by 32 successive power OFF half-cycles, the OFF time is roughly 250 milliseconds. This OFF time allows the element to cool to a point at which the current drawn by the element at the initiation of each sequence of conductive half-cycles is sufficiently large to decrease the reliability of the circuit components over a long period of time. Clearly, this problem is even more severe for the lower duty cycles. Thus, the duty cycle control approach does not provide a satisfactory solution to the steady state current surge problem posed by MoSi.sub.2 type heating elements in applications in which a wide range of power settings is desirable.